1. Field of the Invention
This invention relates to a multilayer printed wiring board for use in making electronic circuits and a process for manufacturing said wiring board.
2. Prior Art
A printed wiring board for use in making electronic circuits has heretofore been manufactured by impregnating a glass cloth with a thermosetting resin such as an epoxy resin and then making the thus impregnated resin semi-cured to obtain a prepreg, placing one copper foil facing to one side of the thus obtained prepreg and another copper foil facing to the other side thereof, pressing the. whole together under pressure and heat by the use of a press to prepare a copper foil-applied laminate, and then subjecting to thus prepared laminate to circuit formation by etching and to perforation.
In this case, there is used a copper foil which has been subjected to so-called unevenness or roughnss formation at one face of the copper foil in order to ensure secure adhesion of the copper foil to the prepreg.
The above copper foil-applied laminate is etched so as to form circuits respectively on both the sides of the laminate thereby to fabricate an inner-layer member. Copper foils are further adhered via the prepregs onto both the sides of the inner-layer member, respectively, and then etched to form outer-layer circuits whereby a multilayer printed wiring board is also manufactured as illustrated in FIG. 1 which is a schematic cross-sectional view of a multilayer printed wiring board having two inner circuits and two outer circuits.
In this case, the inner layer circuits (copper foils) formed in the inner layer member will not fully be adhered to the prepregs of the outer layer member since the exposed surfaces of the inner circuits are smooth surfaces (which are called "glossy face"). Thus, it is a common practice that the surface of the circuits (copper foils) of the inner layer member is subjected to black oxide treatment to ensure the secure adhesion of the inner layer member to the outer layer member and then a multilayer board is fabricated. The adhesiveness of the surface of the inner-layer circuits (copper foils) formed in the inner-layer member respectively to the prepregs of the outer-layer member in greatly enhanced by the black oxide treatment and, thus, the heat resistance and moisture resistance of the resulting multilayer printed wiring board will amount to a practically satisfiable level. The multilayer printed wiring boards produced by the above method have been used in various fields such as electrical and electronic fields.
Although the adhesion of the inner-layer circuits (copper foils) formed in the inner-layer member respectively to the prepregs of the outer-layer member is improved, various problems are raised in other aspects. According to "Technical Manual of Printed Circuits", p. 261, published by Nikkan Kogyo Newspaper Publishing Co.", the various problems include the oxide film is dissolved in hydrochloric acid or the like whereby haloing phenomena are presented and the insulation properties and inter-layer connection reliability are apt to be decreased. The problems also include difficulties in grinding or polishing thin inner-layer circuits (copper foils) and operational difficulties due to the use of a concentrated alkaline solution or the like for the black oxide treatment.
To solve these problems, another method such as one described in, for example, Japanese Patent Gazette No. Hei 4-19306 (or No. 19306/92) has been proposed as a substitute for the black oxide treatment; however, this substitute dose not come to be widely used since it needs specific chemicals and the like. As a method which dispenses with black oxide treatment, there is also known a method using so-called double-treat copper foils both the faces of which have been subjected to roughening treatment. The double-treat copper foils are not generally used since they need a complicated process for being produced, incur a higher cost and raise problem such as insufficient peeling-off of the photoresist therefrom at the time of formation of circuits due to close adhesion of a photoresist to the copper foils.